The present disclosure generally relates to acidizing a subterranean formation, and, more specifically, to acidizing fluids that may minimize the severity of precipitation during an acidizing process.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. Illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations can be used to stimulate a subterranean formation to increase production therefrom. During an acidizing operation, an acid-soluble material in the subterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, or to remove acid-soluble precipitation damage in the subterranean formation. The acid-soluble material being dissolved by the acid(s) can be part of the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g., proppant or gravel particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and aluminosilicates. Other substances can also be dissolved during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo acidization. As further discussed below, certain components dissolved during an acidizing operation can be problematic and possibly detrimental for future production from the subterranean formation.
Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite and dolomite). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the carbonate material. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). Acidizing a siliceous formation or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the mineral and organic acids that can be effective for acidizing a carbonate formation may have little effect on a siliceous formation, since these acids do not effectively react with siliceous materials to affect their dissolution. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during dissolution of a siliceous material. The low pH state may promote initial silicon or aluminum dissolution and aid in maintaining these substances in a dissolved state.
Despite the advantages that can be realized by acidizing a siliceous formation, silicon and aluminum can sometimes further react to produce damaging precipitates after their dissolution that can often be more detrimental for production than if the acidizing operation had not been performed in the first place. The equilibrium solubility levels of silicon and aluminum in a fluid usually depend upon one another. That is, by maintaining high levels of dissolved aluminum during an acidizing operation, silicon dissolution can also be promoted. In this regard, dissolved aluminum can be maintained in a fluid by coordination with fluoride ions, but such aluminum coordination can leave insufficient fluoride ions remaining for effective silicon solubilization to take place, thereby leading to damaging silicon precipitation. Chelating agents can be used to increase the degree of silicon solubilization by maintaining aluminum in a dissolved state. By chelating aluminum to form a soluble aluminum complex, increased levels of dissolved silicon may be realized, since more free fluoride ions are left available to affect its solubilization.
Even when chelating agents are used during an acidizing operation, precipitation can still be problematic. If the amount of aluminum or another metal needing sequestration exceeds the amount of chelating agent that is available, precipitation can still occur. An even more significant issue is that of precipitation of insoluble fluorosilicates and aluminosilicates that can occur in the presence of Group 1 metal ions (i.e., alkali metal ions). The terms “Group 1 metal ions” and “alkali metal ions” will be used synonymously herein. Under low pH conditions (e.g., below a pH of about 3), dissolved silicon can react with Group 1 metal ions (e.g., Na+ and K+) to produce insoluble alkali metal fluorosilicates and aluminosilicates. Other metal ions, including Group 2 metal ions (e.g., Ca2+ and Mg2+), may also be problematic in this regard. In many instances, pre-flush fluids may be introduced to a subterranean formation prior to performing an acidizing operation in order to decrease the quantity of available alkali metal ions. In some instances, such pre-flush fluids can contain ammonium ions (NH4+) that can displace alkali metal ions in the subterranean formation and leave it desirably conditioned for an acidizating operation. In contrast to alkali metal ions, ammonium ions are not believed to promote the formation of insoluble fluorosilicates and aluminosilicates. The use of pre-flush fluids, particularly those containing ammonium ions, can considerably add to the time and expense needed to perform an acidizing operation. In addition to problematic alkali metal ions in the subterranean formation itself, the precipitation of alkali metal fluorosilicates and fluoroaluminates can considerably limit the sourcing of carrier fluids that may be used when acidizing a subterranean formation.
Other techniques can also be applied for mitigating precipitation during an acidizing operation. Without limitation, these additional techniques can include adding agents to a subterranean formation that directly complex silicon or that increase the fluid solubility of highly insoluble substances. Despite the various approaches that can be used for mitigating precipitation during an acidizing operation, precipitation represents a complex problem that can arise from a number of different sources. As a result, precipitation can be a near-inevitable problem that must be addressed in some manner during the course of conducting a downhole operation.